The present invention refers to a method of manufacturing a color CRT having a glass body including a cone and a front shell having side walls and a frit surface.
Before soldering the front shell to the cone of a color CRT, connecting surfaces and the inner surface of the side wall of the front shell are cleaned of graphite conductivity suspension. This cleaning always includes the risk of damaging the coating on the inner front surface side of the front shell.
The cleaning processes are usually carried out during the coating processes. According to a sensible embodiment, the cleaning is performed between applying a graphite suspension and coating with a first fluorescent layer.
Originally, this cleaning was performed mechanically by means of small and big brushes and by means of a cleaning liquid, wherein the device was especially adapted to the contour of the front shell edges. An adaptation of the cleaning method to new screen formats (e.g. 16:9) or different screen sizes was very expensive.
Submerging baths that are used for cleaning, have the advantage that they do not have to be adapted to different screen formats and sizes.
Favorable cleaning results can be achieved by hydrofluoric acid submerging baths. The material most suitable for that purpose is ammonium bifluoride, since the respective diluted solution removes the graphite conductivity suspension and is relatively harmless regarding safety factors. Ammonium bifluoride is therefore used by most of the manufacturers of color CRTs for removing the graphite conductivity suspension.
During cleaning of the connecting surfaces and the inner surfaces of the side walls of the front shell, the graphite conductivity suspension layer is damaged by dashes, whereby rejects are produced.